1. Field of the Invention
Embodiments disclosed herein relate generally to matrix body drill bits and the methods for the manufacture of such drill bits. In particular, embodiments disclosed herein relate generally to use of multiple matrix materials in a bit.
2. Background Art
Various types and shapes of earth boring bits are used in various applications in the earth drilling industry. Earth boring bits have bit bodies which include various features such as a core, blades, and pockets that extend into the bit body or roller cones mounted on a bit body, for example. Depending on the application/formation to be drilled, the appropriate type of drill bit may be selected based on the cutting action type for the bit and its appropriateness for use in the particular formation. In PDC bits, polycrystalline diamond compact (PDC) cutters are received within the bit body pockets and are typically bonded to the bit body by brazing to the inner surfaces of the pockets. The PDC cutters are positioned along the leading edges of the bit body blades so that as the bit body is rotated, the PDC cutters engage and drill the earth formation. In use, high forces may be exerted on the PDC cutters, particularly in the forward-to-rear direction. Additionally, the bit and the PDC cutters may be subjected to substantial abrasive forces. In some instances, impact, vibration, and erosive forces have caused drill bit failure due to loss of one or more cutters, or due to breakage of the blades.
Bit bodies are typically made either from steel or from a tungsten carbide matrix bonded to a separately formed reinforcing core made of steel. While steel body bits may have toughness and ductility properties which make them resistant to cracking and failure due to impact forces generated during drilling, steel is more susceptible to erosive wear caused by high-velocity drilling fluids and formation fluids which carry abrasive particles, such as sand, rock cuttings, and the like. Generally, steel body PDC bits are coated with a more erosion-resistant material, such as tungsten carbide, to improve their erosion resistance. However, tungsten carbide and other erosion-resistant materials are relatively brittle. During use, a thin coating of the erosion-resistant material may crack, peel off or wear, exposing the softer steel body which is then rapidly eroded. This can lead to loss of PDC cutters as the area around the cutter is eroded away, causing the bit to fail.
Tungsten carbide or other hard metal matrix body bits have the advantage of higher wear and erosion resistance as compared to steel bit bodies. The matrix bit generally is formed by packing a graphite mold with tungsten carbide powder and then infiltrating the powder with a molten copper-based alloy binder. The matrix powder may be a powder of a single matrix material such as tungsten carbide, or it may be a mixture of more than one matrix material such as different forms of tungsten carbide. There are several types of tungsten carbide that have been used in forming matrix bodies, including macrocrystalline tungsten carbide, cast tungsten carbide, carburized (or agglomerated) tungsten carbide, and cemented tungsten carbide.
The matrix powder may include further components such as metal additives. Metallic binder material is then typically placed over the matrix powder. The components within the mold are then heated in a furnace to the flow or infiltration temperature of the binder material at which the melted binder material infiltrates the tungsten carbide or other matrix material. The infiltration process that occurs during sintering (heating) bonds the grains of matrix material to each other and to the other components to form a solid bit body that is relatively homogenous throughout. The sintering process also causes the matrix material to bond to other structures that it contacts, such as a metallic blank which may be suspended within the mold to produce the aforementioned reinforcing member. After formation of the bit body, a protruding section of the metallic blank may be welded to a second component called an upper section. The upper section typically has a tapered portion that is threaded onto a drilling string. The bit body typically includes blades which support the PDC cutters which, in turn, perform the cutting operation. The PDC cutters are bonded to the body in pockets in the blades, which are cavities formed in the bit for receiving the cutting elements.
The matrix material or materials determine the mechanical properties of the bit body (in addition to being partly affected by the binder material used). These mechanical properties include, but are not limited to, transverse rupture strength (TRS), toughness (resistance to impact-type fracture), hardness, wear resistance (including resistance to erosion from rapidly flowing drilling fluid and abrasion from rock formations), steel bond strength between the matrix material and steel reinforcing elements, such as a steel blank, and strength of the bond to the cutting elements, i.e., braze strength, between the finished body material and the PDC cutter. Abrasion resistance represents another such mechanical property.
According to conventional drill bit manufacturing, a single matrix powder is selected in conjunction with the binder material, to provide desired mechanical properties to the bit body. The single matrix powder is packed throughout the mold to form a bit body having the same mechanical properties throughout. It would, however, be desirable to optimize the overall structure of the drill bit body by providing different mechanical properties to different portions of the drill bit body, in essence tailoring the bit body. For example, wear resistance is especially desirable at regions around the cutting elements and throughout the outer surface of the bit body while high strength and toughness are especially desirable at the bit blades and throughout the bulk of the bit body. However, unfortunately, changing a matrix material to increase wear resistance usually results in a loss in toughness, or vice-versa.
Further, in packing the matrix powder materials into the mold, the geometry of the bit (and thus mold) make it difficult to place different matrix materials in different regions of a bit because there is little or no control over powder locations in the mold during assembly, particularly around curved surfaces. Previous attempts to pack powders around such geometries were rendered fruitless by the vibration schemes necessary to pack a bit with matrix powder. According to the conventional art, the choice of the single matrix powder represents a compromise, as it must be chosen to produce one of the properties that are desirable in one region, generally at the expense of another property or properties that may be desirable in another region.
Accordingly, there exists a continuing need for developments in matrix bit bodies to improve wear resistance and toughness in the regions of the bit in which these properties are desirable.